Question

(a) Do you expect the viscosity of glycerol, \(\mathrm{C}_{3} \mathrm{H}_{5}(\mathrm{OH})_{3}\), to be larger or smaller than that of 1-propanol, \(\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{OH}\) ? (b) Explain. [Section 11.3]

Short answer

(a) The viscosity of glycerol is expected to be larger than that of 1-propanol. (b) This is due to glycerol having three hydroxyl groups per molecule, allowing it to form more hydrogen bonds with neighboring molecules. This results in stronger intermolecular forces and a higher viscosity compared to 1-propanol, which only has one hydroxyl group per molecule.

Step-by-step solution

Understanding viscosity

Viscosity is a measure of a liquid's resistance to flow. It depends on the strength of the intermolecular forces between the liquid's molecules. Stronger intermolecular forces lead to higher viscosities because the molecules are more attracted to each other, making it more difficult for the liquid to flow.

Examining the molecular structures of glycerol and 1-propanol

The molecular structures of glycerol and 1-propanol play a crucial role in determining their viscosities. Glycerol: \(\mathrm{C}_{3} \mathrm{H}_{5}(\mathrm{OH})_{3}\) 1-Propanol: \(\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{OH}\) As we can see, both molecules have the same number of carbon atoms, but glycerol has three hydroxyl groups \((\mathrm{OH})\), while 1-propanol has only one hydroxyl group.

Analyzing intermolecular forces

Intermolecular forces are the forces of attraction or repulsion between molecules. The primary intermolecular force in both glycerol and 1-propanol is hydrogen bonding due to the presence of hydroxyl groups. In glycerol, there are three hydroxyl groups per molecule, which can form more hydrogen bonds with other glycerol molecules. On the other hand, 1-propanol has only one hydroxyl group per molecule, which can form fewer hydrogen bonds compared to glycerol.

Comparing the viscosities of glycerol and 1-propanol

Since glycerol has more hydroxyl groups and can form more hydrogen bonds with other molecules, its intermolecular forces are stronger than those in 1-propanol. Stronger intermolecular forces lead to higher viscosity. (a) We expect the viscosity of glycerol to be larger than that of 1-propanol. (b) This is because glycerol has more hydroxyl groups per molecule, which can form more hydrogen bonds with other glycerol molecules, resulting in stronger intermolecular forces and a higher viscosity compared to 1-propanol.

Key concepts

Understanding Intermolecular Forces

When studying the characteristics of liquids, it is essential to grasp the concept of intermolecular forces. These are the forces that act between separate molecules, holding them together and determining many of the liquid’s properties, such as boiling and melting points, evaporation rate, and, importantly, viscosity.

There are several types of intermolecular forces, including London dispersion forces, dipole-dipole interactions, and hydrogen bonds. London dispersion forces are the weakest and are present in all molecules, caused by the temporary uneven distribution of electrons that create an instantaneous dipole. Dipole-dipole interactions occur between polar molecules, that is, molecules with a permanent dipole moment, where the positive end of one molecule attracts the negative end of another.

The strongest of these forces, hydrogen bonding, significantly impacts the viscosity of a liquid. The strength of these interacting forces is dependent on the structures of the individual molecules and the ability of those structures to interact with one another.

The Role of Hydrogen Bonding

A special type of intermolecular force, hydrogen bonding, occurs when a hydrogen atom bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine, is attracted to another electronegative atom in a different molecule. This bond is relatively strong compared to other intermolecular forces and has a profound effect on physical properties. For example, water is a liquid at room temperature largely due to hydrogen bonds between its molecules, whereas other molecules of similar size and molecular weight, but without hydrogen bonds, are gases under the same conditions.

In the context of viscosity, hydrogen bonding can greatly increase a liquid’s resistance to flow. More hydrogen bonds mean the molecules are more tightly 'stuck' together, requiring more energy to move apart and thus, creating a higher viscosity. Glycerol's three hydroxyl groups allow for multiple hydrogen bonds, significantly enhancing its cohesive forces and leading to a greater viscosity than in substances with fewer hydrogen bonding opportunities, such as 1-propanol.

Influence of Molecular Structure on Viscosity

A molecule's molecular structure directly influences its physical characteristics, including viscosity. The molecular structure determines how molecules pack together, the type and number of intermolecular forces present, and the molecule’s polarity - all of which play a role in how a liquid flows.

Considering the molecular structures of glycerol and 1-propanol, both consist of three carbon chains, but glycerol's structure includes three hydroxyl groups. These groups are areas of high electron negativity and are prime sites for hydrogen bonding. Since glycerol molecules can form more hydrogen bonds than 1-propanol molecules, this leads to a higher viscosity because the liquid's molecular 'glue' is much stronger, resisting flow more forcefully. Therefore, the structural aspect of the multiple hydroxyl groups in glycerol is the main reason behind its higher viscosity compared to 1-propanol, which has a simpler structure with fewer sites for hydrogen bonding.